The present invention relates to a liquid crystal display. More particularly, the present invention provides an In-plane switching (IPS) type liquid crystal display, in which an electric field is formed laterally to its substrates and applied to a liquid crystal layer, having a wide viewing angle, a high contrast ratio and an excellent display quality.
In a liquid crystal display interposing a liquid crystal layer between a pair of substrates, at least one of them being transparent, some display mode have been used as a display method. Examples of the display mode which have widely been used include a twisted nematic (TN) mode. According to this mode, an electric field is applied to a liquid crystal layer to control its rotary polarization, thereby controlling a transparency of the liquid crystal layer and obtaining display of image.
On the other hand, there has been an in-plane switching (IPS) mode for applying, to a liquid crystal, an electric field which is almost parallel with a substrate by using interdigital shaped electrodes (hereinafter referred to as an interdigital electrode) or the like. According to this mode, an electric field is applied to the liquid crystal to control birefringence of the liquid crystal, thereby switching display of images. Japanese Patent No. 2743293 has disclosed that the IPS mode has a greater angle of view characteristic than that in a conventional TN mode.
FIG. 9 is an explanatory view showing a part of a conventional liquid crystal display using an IPS mode which is formed by a liquid crystal having a positive dielectric anisotropy. FIG. 9 shows an electrode substrate 51, two kinds of interdigital pixel electrodes 53 and 54 formed on the electrode substrate 51, a counter substrate 52, and liquid crystal molecules included in a liquid crystal layer 60 which constitute a liquid crystal panel in the liquid crystal display. As shown in FIG. 9, the electrode substrate 51 having the interdigital pixel electrodes 53, 54 and the counter substrate 52 are provided in parallel with each other and the liquid crystal layer 60 including the liquid crystal molecules is present between the electrode substrate 51 and the counter substrate 52. As shown in FIG. 10(a), the electrode substrate 51 and the counter substrate 52 are subjected to an alignment treatment in a direction of angle 55, thereby the liquid crystal molecules of the liquid crystal layer 60 are provided to have an angle of xcex81c with respect to a length-wise direction of pixel electrode 53 and 54.
Next, a display principle of the liquid crystal display will be described with reference to FIG. 11. FIG. 11 is a partial view showing the liquid crystal display using the IPS mode. FIG. 11 shows only the liquid crystal panel and two polarizing plates in the liquid crystal display. Referring to the liquid crystal panel, particularly, there are shown the electrode substrate 51, interdigital pixel electrodes 53 and 54 (only one pixel electrode is shown respectively), the counter substrate 52 and liquid crystal molecules included in the liquid crystal layer 60 (only seven molecules are shown). In FIG. 11, the reference numeral 53 denotes a first pixel electrode, the reference numeral 54 denotes a second pixel electrode, the reference numeral 51 denotes an electrode substrate, the reference numeral 52 denotes an counter substrate, the reference numeral 59 denotes a first polarizing plate and the reference numeral 58 denotes a second polarizing plate. As shown in FIG. 11, the first polarizing plate 59 is provided such that a direction of a major axis of the liquid crystal molecule is parallel with a direction of a transmission axis of the first polarizing plate 59 (show in a double headed arrow) and a direction of a transmission axis of the second polarizing plate 58 (also shown in a double headed arrow) is orthogonal to that of the first polarizing plate 59. Directions of aligning treatments of alignment films (not shown) formed on the electrode substrate 51 and the counter substrate 52 are both parallel with the transmission axis of the first polarizing plate 59 or the second polarizing plate 58.
In a state in which no voltage is applied (that is, an electric field is not formed between the first pixel electrode 53 and the second pixel electrode 54 as shown in FIG. 11A), a linearly polarized light incident on the liquid crystal layer 60 has an oscillation direction parallel with the liquid crystal molecule and does not receive a birefringence effect during passage through the liquid crystal layer. Therefore, a direction P of oscillation of the light passing through the counter substrate 52 is orthogonal to the transmission axis of the second polarizing plate 58 and the light transmitted through the counter substrate 52 cannot be transmitted through the second polarizing plate 58 and is set in a dark state.
In a state in which voltage is applied (that is, the electric field is formed between the first pixel electrode 53 and the second pixel electrode 54) as shown in FIG. 11B, the liquid crystal molecule is rotated in the direction of the electric field (the degree of the rotation depends on a magnitude of the electric field) while maintaining a parallel orientation with respect to the surfaces of the electrode substrate 51 and the counter substrate 52. For this reason, the linearly polarized light incident on the liquid crystal layer 60 receives the birefringence effect to be changed into an elliptically polarized light Q and a certain quantity of the light passes through the second polarizing plate. The quantity of the light transmitted through the second polarizing plate is changed depending on a rotating angle xcex8 of the liquid crystal molecule. The rotating angle xcex8 of the liquid crystal is a function of an applied voltage (V). Thus, display of image can be carried out by changing voltages to be applied to the first pixel electrodes 53 and the second pixel electrodes 54.
At this time, an intensity of the transmitted light is expressed in the Equation 1:
I=Ioxc2x7sin2(xcfx80R/xcfx80)xc2x7sin2(2xcex8)xe2x80x83xe2x80x83(Equation 1) 
Wherein Io represents an intensity of the light incident on the first polarizing plate 59, xcex represents a wavelength of the light, and R represents a retardation which is represented by an optical path difference (xcex94n)xc2x7d between an ordinary light and an extraordinary light, xcex94n representing an absolute value (|nexe2x88x92no|) of a difference between a refractive index no of the ordinary light and a refractive index ne of the extraordinary light in the liquid crystal. As is apparent from the Equation 1, the transmitted light has a maximum intensity with xcex8=xcfx80/4.
FIG. 12 shows a change in a quantity of the transmitted light with a variation in a voltage to be applied between the first pixel electrode 53 and the second pixel electrode 54. When the applied voltage is increased, the rotating angle of the liquid crystal molecule becomes greater and the quantity of the transmitted light is increased. xcex8 in the Equation 1 corresponds to the average aligning direction of the liquid crystal layer realigned by an electric field formed laterally to the substrate.
Usually, the TN mode is used in a normally white mode in which white display is carried out without the application of an electric field and black display is carried out with the electric field applied. At this time, more black display can be obtained by the application of a higher electric field to the liquid crystal. As a result, a high contrast ratio can be achieved.
On the other hand, the IPS mode is used in a normally black mode in which the black display is carried out without the application of the electric field and the white display is carried out with the electric field applied. FIGS. 9 and 10 show sectional and plan views showing a conventional IPS mode, respectively. In a liquid crystal display using an IPS mode, xcex8=0 is theoretically obtained in the Equation 1 without the application of the electric field so that light is not transmitted. However, the pixel electrodes 53 and 54 actually have certain thickness. Therefore, when an aligning treatment such as rubbing is performed, a region 62 having an aligning direction shifted from the angle 55 of the aligning treatment as well as a region 61 having an aligning direction almost coincident with the angle 55 of aligning treatment is formed.
By the region 62 having the aligning direction shifted, transmission of light is caused in spite of the black display. This is a serious problem when a high contrast ratio is to be implemented by using the IPS mode as a practical liquid crystal display. Moreover, in the case in which this region is generated in a rubbing direction, it is visible as a stripe-shaped mura having a varied brightens. Therefore, there is a problem in respect of display quality.
In particular, in the IPS mode liquid crystal display equipped with active switching elements such as TFTs (thin film transistors), on the substrate there are electrodes for applying the lateral electric field to the liquid crystal, active switching elements for selectively applying voltages to the electrodes, and signal wirings for applying electric signals to the switching element as well as the electrodes. That is, surface irregularities of several hundreds to several thousands nm (nanometer) caused by the electrodes, switching elements and wirings are formed on the substrate. Furthermore, in which a color filter is formed on the counter substrate in order to carry out color display, colorants contained in the filter, seams formed between pixels, a black matrix, and the like cause surface irregularities on the counter substrate. These irregularities on the substrates make an uniform alignment of the liquid crystal to be disturbed, and therefore problematic.
Japanese Unexamined Patent Publication No. 333151/1998 has disclosed a method preventing the generation of said stripe-shaped mura caused by deflected distribution of rubbing cloth staples. In this method, projections to fix the deflected distribution of rubbing cloth staples are formed on the periphery of the substrate. However, the detailed conditions of rubbing process are not described.
Furthermore, it is also possible to form or widen a black matrix on the counter substrate in order to block the light passing through the region in which aligning direction of liquid crystal is shifted. With the black matrix, undesired leakage of light is prevented, so that high contrast ratio is obtained. However, an aperture ratio of the substrate (i.e. that of liquid crystal panel) is decreased with the black matrix, thereby a luminance of white image is reduced.
Moreover, a contrast ratio is greatly affected by a relationship between the aligning direction of the liquid crystal on two interfaces where the liquid crystal and each substrate come in contact with each other and an absorption axis of a polarizing plate provided on the outside of each substrate.
In order to solve the above-mentioned problems, the present invention has been made.
As shown in FIG. 1, a liquid crystal display according to the present invention comprises a pair of substrates 1, 2 and a liquid crystal layer 5 between the substrates. One of said substrates is provided with pixel electrodes 3, opposed electrodes 4 and active switching elements (not shown) to apply electric fields, which are substantially parallel to the substrate, to liquid crystal molecules included in the liquid crystal layer 5. Further, the liquid crystal display of the present invention comprises optical devices (e.g. polarizing plates) 7, 8 to selectively pass the light. Furthermore, either or both of the substrates 1, 2 are provided with alignment films 6a, 6b on their surface facing the liquid crystal layer, thereby aligning directions of liquid crystal molecules in each pixel are set to be in a range of xc2x12 degree with respect to an average aligning direction xcex81c of the liquid crystal layer 5 without applying electric fields.
The liquid crystal layer 5 designates liquid crystal molecules interposed between the substrates 1, 2. Moreover, the average aligning direction xcex81c of the liquid crystal layer, which is shown in FIG. 8 diagrammatically, is computed with following equation 2.
xcex81c=xcexa3(xcex8ixc3x97Si)/S(i=1, 2, 3 . . . )xe2x80x83xe2x80x83(Equation 2) 
In the equation, xcex8i designates an aligning direction of liquid crystal, Si designates an area of regions having the aligning direction of xcex8i, and S designates an area of effective display region through which the light transpire to display images. It should be noted that separators, such as polymer beads, for maintaining prescribed distance between two substrates may be dispersed into the liquid crystal layer, regions around the separators in which aligning directions are varied are excluded from the area S.
That is, the average aligning direction xcex81c of the liquid crystal layer is computed with following steps. Firstly, the effective display region is divided into the regions according to their aligning direction xcex81, xcex82, xcex83, . . . . Then, the area S1, S2, S3, . . . of each region is multiplied by its aligning direction xcex81, xcex82, xcex83, respectively, and products xcex81xc3x97S1, xcex82xc3x97S2, xcex83xc3x97S3, . . . are summed up. Lastly, the sum of the products is divided by the area S of the effective display region, therefore the average aligning direction xcex81c is obtained.
Here, an aligning direction xcex8i of liquid crystal in each region is defined and measured as follows. A pair of substrates sandwiching a liquid crystal layer is interposed between two polarizing plates having their absorption axes orthogonal to each other (i.e. cross nicole arrangement). When the liquid crystal layer with the substrates is rotated in relative to the polarizing plates, the intensity of light passing through the liquid crystal layer and the polarizing plates varies. Therefore, an angle between the absorption axis of one polarizing plate and length-wise direction of pixel electrodes on the substrate wherein the intensity of light passing through the region becomes minimized is assumed as aligning direction xcex8i of liquid crystal in this region.
With referring to FIGS. 7(a) to 7(c), an aligning direction of liquid crystal in one region is explained. In an example shown in FIG. 7(a), a liquid crystal molecule on the electrode substrate and a liquid crystal molecule on the counter substrate as well as the other liquid crystal molecules have almost same aligning direction, i.e. xcex81c. On the other hand, in an example shown in FIG. 7(b), an aligning direction of a liquid crystal molecule on the electrode substrate is not equal to that of a liquid crystal molecule on the counter substrate, however an average aligning direction in this region is xcex81c. FIG. 7(c) shows an alignment of the liquid crystal in which each liquid crystal molecule is realigned under application of electric field by the pixel electrodes, wherein xcex81c corresponds to an angle at which a quantity of transmitted light, i.e. light passing through the liquid crystal and the polarizing plates sandwiching the liquid crystal and having their axes orthogonal to each other, is minimized.
Moreover, it is preferable that the alignment film formed of an organic substance should be used for controlling the orientation of the liquid crystal. In order to relieve the irregularities on a surface of the substrate, particularly of a substrate on which active switching elements are formed, so as to obtain an uniform aligning direction, it is effective to flatten the irregularities of the substrate by means of an insulating materials. To form an aligning film thickly is also effective to relieve the irregularities. Furthermore, a flattened layer may be provided on the surface of the substrate and an aligning film may be formed on the flattened layer.
The alignment treatment of the liquid crystal is carried out by a rubbing method, and it is preferable that a rubbing strength L shown in Equation 3 should be 50 mm or more.
L=Nxc3x97ld(1+2xcfx80rn/(60V))xe2x80x83xe2x80x83(Equation 3) 
wherein N represents the number of times of rubbing [number], ld represents an amount of deformation of rubbing cloth [mm], r represents a radius of a rubbing roller [mm], n represents the number of revolution of the roller per minutes [rpm] and V represents a stage moving speed [mm/s].
With reference to FIG. 6, a rubbing process for aligning the liquid crystal will be described. As shown in FIG. 6, a roller 72 on which a velvet-like rubbing cloth 73 made of rayon or cotton and having a small staple length is wound is rotated at n times/min. in a direction of an arrow R. At the same time, a substrate 74 having an alignment film formed is mounted on a stage 71 and is moved at a constant speed V in a direction of an arrow S. The rubbing cloth 73 on the roller 72 is pushed against the substrate 74 on the stage 71 and deformed, therefore the deformation of the rubbing cloth is assumed to an amount of deformation ld. A rubbing direction of the substrate 74 thus rubbed is represented by a composite vector of a speed vector at which the rubbing cloth 73 rubs the substrate 74 and a moving speed vector of the stage 71. In general, the speed at which the rubbing cloth 73 rubs the substrate 74 is much higher than the moving speed of the stage 71. Therefore, the rubbing direction is obtained by projecting the rotating direction of the rubbing roller onto the substrate.
In some cases in which a surface of the substrate is flattened, an effective aligning treatment can be achieved even if the rubbing strength L is set to 50 mm or less. However, since the flattening process causes an increase in the number of manufacturing steps, a decrease in an effective voltage to be applied to a liquid crystal and a rise in cost, it is not preferable. On the other hand, if the rubbing strength L is increased, mura in a displayed image appears easily. Therefore, it is preferable that the rubbing strength L should be 300 mm or less.
Moreover, it is preferable that the aligning direction of the liquid crystal molecule in the interface of one substrate and that of the liquid crystal molecule in the interface of the other substrate should be almost equal to each other. The expression of xe2x80x9calmost equalxe2x80x9d implies that an angle defined by both aligning directions is 0 to 3 degrees.
In the present invention, moreover, it is preferable that an optical device for selectively passing the light according to the alignment of the liquid crystal molecules should be a pair of polarizing plates provided on the outside of the substrates respectively and absorption axes of the polarizing plates should be almost orthogonal to each other. The expression of xe2x80x9corthogonalxe2x80x9d implies that the angle formed by the absorption axes of the polarizing plates ranges from 85 degrees to 95 degrees. More preferably, the angle ranges from 88 degrees to 92 degrees.
Moreover, it is preferable that the relationship between a direction xcex8p1 of the absorption axis of the polarizing plate closer to an observer and the average aligning direction xcex81c of the liquid crystal should be set to |xcex81c|xe2x88x922+xe2x89xa6|xcex8p1|xe2x89xa6|xcex81c|+3xc2x0 or the relationship between a direction xcex8p2 of the absorption axis of the polarizing plate which is more distant from the observer and the average aligning direction xcex81c of the liquid crystal should be set to |xcex81c|xe2x88x922xc2x0xe2x89xa6|xcex8p2|xe2x89xa6xcex81c|+3xc2x0. At this time, when the liquid crystal panel is seen from the substrate 1 side, xcex81c, xcex8p1 and xcex8p2 represent, as a positive direction, a rotation in which the liquid crystal molecule is rotated by applied electric field.
Furthermore, it is preferable that the absorption axis xcex8p1 of one of the polarizing plates and the average aligning direction xcex81c(bk) of the liquid crystal during black display are almost equal to each other. The expression of xe2x80x9calmost equalxe2x80x9d implies that an angle formed between the absorption axis and the average aligning direction is 0 to 3 degrees.
It is preferable that the aligning direction of the liquid crystal molecules in each pixel should range within xc2x12 degrees with respect of the average aligning direction xcex81c of the liquid crystal layer.